1. Field of the Invention
This invention relates to a still-video camera (also referred to as an electronic still camera) capable of photography using a shutter.
2. Description of the Related Art
By making use of a shutter in a still-video camera, it is possible to control the storage time of an electric charge in an electronic image pick-up device. Since storage of electric charge in an image pick-up device is carried out while the shutter is open, read-out from the pick-up device is inhibited during this time. When the shutter is closed, the electric charge is read out of the image pick-up device at a predetermined timing and the read still-video signal is recorded on a video floppy (magnetic disk).
Conventionally, read-out is inhibited using a shutter control signal. That is, the control signal is such that a leading blind of the shutter is released and made to travel at the leading edge of the signal and a trailing blind is released and made to travel at the trailing edge of the signal. In the conventional image pick-up devices, read-inhibit is cancelled at the trailing edge of the control signal. However, a problem which arises is that since the shutter is still not fully closed at the trailing edge of the control signal, non-uniform exposure develops on the recorded frame of the video signal when the signal is read out of the image pick-up device.
Accordingly, it is preferred that read-out from the image pick-up device be inhibited until the shutter is completely closed. Full closure of the shutter can be sensed by a sensor or by clocking elapsed time from the moment the latch of the trailing blind is released. Therefore, cancellation of read-inhibit after the shutter is completely closed is capable of being realized with comparative ease.
An important problem which must be considered in determining the moment at which read-inhibit is cancelled is the fact that there is a phase difference between a phase pulse PG which decides the start of write-in to the video floppy and a field-shift pulse FS which controls the read-out of the video signal from the image pick-up device. As is well known, the phase pulse PG is generated at every revolution of a video floppy. In a still-video camera, the video signal, which represents the subject photographed, is written in the video floppy during one revolution of the video floppy, namely between one phase pulse PG and the next phase pulse PG. The rotation of the video floppy is controlled in such a manner that the phase pulse PG will be generated at a position which is 7H.+-.2H ahead of a vertical synchronizing signal, and the field-shift pulse FS is outputted with a constant phase delay with respect to the vertical synchronizing signal.
As mentioned above, read-out from the image pick-up device by the field-shift pulse FS is inhibited while the shutter is open. In addition, write-in to the video floppy is performed from the moment of the next phase pulse PG following cancellation of read-inhibit. If the inhibition on read-out from the electronic image pick-up device were to be cancelled between the phase pulse PG and the field-shift pulse FS, read-out of the video signal from the pick-up device by the field-shift pulse FS would be carried out, but it would not be possible to carry out recording without waiting for the next phase pulse PG. In other words, most of the read video signal would not be written in the video floppy.